Keeping the Alaskan Way Viaduct safe until the tunnel opens

We're looking forward to opening the tunnel to traffic and demolishing the Alaskan Way Viaduct. It’s the reason we come to work each day. But until we take down the viaduct, it’s our job to protect it and keep it safely open to drivers.

And so this weekend, as they do four times every year, WSDOT crews will inspect the viaduct. They’ll start by closing the structure to traffic on Saturday and Sunday. Then they’ll begin a methodical weekend of work measuring existing cracks, looking for new ones, checking for structural movement and evaluating the integrity of the viaduct’s foundations. When their work is done, the structure will reopen to traffic, just as it has following the dozens of viaduct inspections we’ve performed since the Nisqually earthquake struck Seattle in 2001.

These inspections are an essential part of our core mission: public safety. The viaduct remains safe for everyday use, and it will continue to provide a vital link to and through downtown Seattle until the SR 99 tunnel opens to traffic. But it remains vulnerable to earthquakes like the one that set this whole project in motion.

What we've done to protect the viaduct
We’ve done a significant amount of work to reinforce and maintain the viaduct over the years. Immediately after the Nisqually quake, we repaired damaged support columns and expansion joints to make the structure safe and functional. Additionally, vehicles weighing more than 105,500 lbs. are prohibited and trucks and buses must travel in the right-hand lane only to limit the number of heavy vehicles on the viaduct in one location at one time. In 2008, we strengthened four viaduct columns between Columbia Street and Yesler Way that had continued to settle despite the initial repairs.

As discussion about how to replace the waterfront section of the viaduct continued, we began work on a system designed to close the viaduct automatically in the event of a moderate to severe earthquake in the greater Seattle area. Completed in 2011, the automated closure system consists of traffic gates at all viaduct access points controlled by an earthquake detection system. If the earthquake monitoring system detects significant ground movement, it will simultaneously lower all nine traffic gates and safely close the viaduct in two minutes.

When the bored tunnel was selected as the preferred option and it was determined it would pass beneath the viaduct, we made strengthening portions of the structure near the tunnel route a requirement of the tunnel contract. Our contractor, Seattle Tunnel Partners (STP), has since implemented those requirements.

As part of that work, crews built underground walls to protect the viaduct's foundations at key locations and wrapped sections of the structure in carbon fiber to provide additional strength. Additionally, more than 100 monitors were installed on the viaduct to measure ground movement beneath the structure in real time.

Interpreting settlement data as construction continues
Not all settlement is significant. In the case of the viaduct, no one number represents an acceptable level of settlement. Those limits vary along the length of the viaduct based on ground conditions and the condition of the structure.

How the ground settles is also important. A structure that settles uniformly is less likely to be damaged than a structure that settles unevenly. So far during tunnel construction, our monitors have told us that the viaduct has settled up to four-tenths of an inch at two locations near construction. This is well within acceptable levels established in STP's design. We will continue to carefully monitor the viaduct during construction and will take additional steps to reinforce the structured if needed.

The viaduct will continue to close temporarily for inspections, and occasionally for construction. That includes a one- to two-week closure so the tunneling machine can pass beneath the structure and other closures that will be necessary to connect the new tunnel with existing sections of SR 99 at the north and south ends of downtown. Beyond that, we have no plans to close the viaduct -- as some recent media reports have suggested -- until after the tunnel opens to traffic.